1. Field of the Inventions
The present inventions relate to a mounting structure of an anode for corrosion prevention (sacrificial anode) for a marine engine.
2. Description of the Related Art
When an outboard motor is employed on the sea, its engine, which may be made of an aluminum alloy, is corroded by sea water used as coolant for the engine. To cope with this problem, anodes made of a metal such as zinc, with a lower polarization potential than an aluminum alloy, have been disposed in the coolant passages of such engines. As such, the anode acts as a sacrificial anode, thereby preventing the electrochemical reaction causing electrolytic corrosion of the engine. For example, see Japanese Patent Publication Nos. Hei 10-236390 and Hei 11-11390.
The electrolytic corrosion prevention effect of such anodes decreases with increasing distances from the mounting position of the anode. Thus, a satisfactory electrolytic corrosion prevention effect can be obtained only within a limited range, which can also vary depending on the material used for the sacrificial anode and the cross-sectional area of the coolant passage. Therefore, to achieve a satisfactory electrolytic corrosion prevention effect with an anode, multiple anodes are often attached throughout the coolant passage at given intervals. For example, some engines are constructed with anodes spaced apart by about 300 mm or less for a coolant jacket of an aluminum alloy, four-cylinder engine having a displacement of about 1800 cubic centimeters.
However, in such a conventional anode mounting structure, it is difficult in practice to attach anodes at given intervals inside a coolant jacket, which can have a complicated shape, such as those formed around the combustion chambers of en engine. Additionally, areas can be found at which no effect of the anodes is produced, thereby allowing the engine to corrode in those areas.
Additionally, in order to mount anodes to an engine body so as to be removable, leak-proof, and in communication with fluids in the cooling jacket, mounting seats are machined into the engine body. The seats are formed, for example, in the wall surface of the cylinder head or the cylinder body and the anodes are attached to the seat faces. However, such seats increase the size of the engine. Additionally, such machining and assembling can be time-consuming and thus can lower productivity.
Fixing the anode to the seats requires a reliable fastening method which withstands engine vibration in corrosive environments and which ensures an electrical connection. Thus, the seats are typically provided with machine threads so that the anodes can be secured with tight fitting bolts.
One alternative approach, described in Japanese Patent Publication No. Hei 6-11042, includes using a cylinder head gasket having an inner core with an electrolytic solution potential approximately equal to that of the cylinder head and the cylinder body. As such, the gasket can suppress the electrolytic corrosion of the engine. However, this gasket has a special and complicated structure of three layers, causing trouble in manufacturing.